High temperature superconducting dielectric resonator having mode absorbing means

ABSTRACT

A dielectric resonator apparatus for measuring the parameters of high temperature superconducting thin film is disclosed having improved means for positioning the dielectric and substrates, holding the resonator components in place during use, suppressing undesirable modes, adjusting the magnetic dipole coupling, and coupling to an electrical circuit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Ser. No. 07/934,144filed Aug. 21, 1992, now abandoned.

FIELD OF THE INVENTION

This invention relates to an apparatus which is used to measure hightemperature superconducting thin film parameters such as surfaceresistance, critical current density, and critical magnetic field.

BACKGROUND OF THE INVENTION

The quality of superconductor thin films is described by severalparameters: critical temperature, T_(c), surface resistance, R_(s),critical current density, J_(c), critical magnetic field, H_(c), etc.For microwave and millimeter wave applications, the most importantparameter of a superconductor film is the surface resistance, R_(s), ata given frequency, measured as a function of temperature, currentdensity (or rf magnetic field). Measurement of these parametersaccurately is not only necessary for superconductor material researchand applications, but is also important for controlling the quality ofmanufacturing superconducting film.

One method for measuring surface resistance is called "TE₀₁₁ mode cavityend wall replacement" Muller et al., J. Superconductivity, Vol. 3, p.235-242 (1990). It utilizes a copper cylindrical cavity operating atTE₀₁₁ mode with one of its two end walls replaced by a superconductorfilm. The R_(s) of the film can be determined by comparing the Q-valuesof the cavity with the sample to the same cavity with a calibrationstandard film (such as niobium or copper) having a known R_(s) value.This method has the following shortcomings: 1) it requires calibration,so it is not an absolute measurement; 2) the accuracy is limited by thefact that the R_(s) of the sample film under test only contributes asmall portion of the loss in the cavity; 3) the measurable range of theR_(s) is limited at the low end by the poor sensitivity of this method.

Another method for measuring R_(s) is called "parallel plate resonator"as disclosed in Taber, R., Rev. Sci. Instrum., Vol. 61, p. 2200-2206(1990). It is constructed by two pieces of superconducting filmseparated by a thin dielectric spacer. The R_(s) of the superconductorfilm can be determined by measuring the Q-value of the resonator. Thismethod has the following shortcomings: 1) because the spacer is verythin, it is very difficult to couple the rf power in and out of theresonator; 2) since the Q-value is relatively low, the measurable rangeof R_(s) is limited at the high end by a weak coupling; 3) since theparallel plate resonator is an open structure, the rf magnetic field isnot confined, which results in poor accuracy and case mode interference;4) it is not an absolute method, calibration is required.

Yet another method is called "dielectric resonator" as disclosed byFiedziusko et al., IEEE-MTT-S International Microwave Symposium Digest,Vol. 2, p. 555-558 (1989) and by Llopis et al., J. Less-Common Metals,Vols. 164, 165, p. 1248-1251 (1990). There are two different versions.One version involves putting a dielectric resonator on top of the samplesuperconducting film under testing. Again the R_(s) of the film can bedetermined by measuring the Q-value of the resonator. This method hasthe following shortcomings: 1) the open structure makes it difficult toconfine the rf fields, which results in poor accuracy and modingproblems; 2) means for holding the dielectric resonator in the rightplace is a problem. The second version is asuperconductor-dielectric-superconductor sandwich. Adding the secondsuperconductor film solved the problems encountered in use of the firstversion. However, since the dielectric material used has a poor losstangent factor, the sensitivity of this method is limited.

Currently available apparatus for measuring surface resistance are notsuitable for use as a production tool because of their limitations.Films cannot be tested at high power. Measurement is not accurate, andits reproducibility is poor. The dynamic range is limited. The assemblyis time consuming. Finally, the measurement is very sensitive to how thefilms are assembled in the apparatus. Thus there is a need for anapparatus suitable for use in quality control operations for monitoringsuperconducting film manufacturing processes.

The present invention provides apparatus suitable for improvedcharacterization of high temperature superconducting thin films. Majorimprovement in performance has been achieved as well as the ability touse the same concept design to make the resonators that can characterizedifferent sizes of superconducting thin films.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) is a top view of resonator #1 with the top plate removed.

FIG. 1(b) is an exploded cross-sectional view of resonator #1.

FIG. 1(c) is a cross-sectional view of a film of a substrate coated withsuperconducting film.

FIG. 1(d) illustrates the film-sapphire-film structure.

FIG. 1(e) is an cross-sectional view of the connector assembly.

FIG. 2(a) illustrates the design of drawer #1 of resonator #1.

FIG. 2(b) is a front view of drawer #1 of resonator #1.

FIG. 2(c) is a cross-sectional view of drawer #1 of resonator #1.

FIG. 2(d) is a top view of sapphire locater #1 of resonator #1.

FIG. 2(e) is an enlarged view of arc 125 of sapphire locator #1 ofresonator #1.

FIG. 2(f) illustrates the design of drawer #2 of resonator #1.

FIG. 2(g) is a front view of drawer #2 of resonator #1.

FIG. 2(h) is a cross-sectional view of drawer #2 of resonator #1.

FIG. 2(i) is a top view of sapphire locator #2 of resonator #1.

FIG. 3(a) illustrates the design of the body of resonator #1.

FIG. 3(b) is a front view of the body of resonator #1.

FIG. 3(c) is a bottom view of the body of resonator #1.

FIG. 3(d) shows holes for fastening connectors onto the body ofresonator #1.

FIG. 3(e) illustrates the design of the top plate of resonator #1.

FIG. 3(f) is a side view of the top plate of resonator #1.

FIG. 3(g) illustrates the design of the bottom plate of resonator #1.

FIG. 3(h) is a side view of the bottom plate of resonator #1.

FIGS. 3(i) and 3(j) illustrate the design of the transition of resonator#1.

FIGS. 3(k) and 3(m) illustrate the design of the insulator of resonator#1.

FIG. 4(a) illustrates the design of the piston of resonator #1.

FIG. 4(b) is a front view of the piston of resonator #1.

FIG. 4(c) is an enlarged view of a portion of the piston of resonator#1.

FIG. 4(d) is a cross-sectional view of the piston of resonator #1.

FIG. 4(e) illustrates the design of the piston locater of resonator #1.

FIG. 5(a) is a top view of sapphire locating device #3.

FIG. 5(b) illustrates the design of sapphire locating device #4.

FIG. 5(c) is a bottom view of platform (223) of sapphire locating device#4.

FIG. 5(d) is a side view of platform (223) of sapphire locating device#4.

FIG. 6(a) is a cross-sectional view of holding device #1.

FIG. 6(b) is a cross-sectional view of holding device #2.

FIG. 6(c) is a cross-sectional view of holding device #3.

FIG. 7(a) is a top view of resonator #2 with the top plate removed.

FIG. 7(b) is an exploded cross-sectional view of resonator #2.

FIG. 7(c) is an cross-sectional view of the connector assembly.

FIG. 7(d) is a cross-sectional view of a film of a substrate coated withsuperconducting film.

FIG. 7(e) illustrates the film-sapphire-film structure.

FIG. 8(a) illustrates the design of the body of resonator #2.

FIG. 8(b) illustrates tapped hole (315) in the side of the body ofresonator #2 for screw sub-assemblies (260).

FIG. 8(c) illustrates holes used for attachment of connectors of theside of the body of resonator #2.

FIG. 8(d) illustrates the design of the top plate of resonator #2.

FIG. 8 (e) is a front view of the top plate of resonator #2.

FIG. 8(f) illustrates the design of the bottom plate of resonator #2.

FIG. 8(g) is a cross-sectional view of the bottom plate of resonator #2.

FIG. 9(a) illustrates the design of the piston of resonator #2.

FIG. 9(b) is a cross-sectional view of the piston of resonator #2.

FIG. 9(c) is a bottom view of the piston of resonator #2.

FIG. 9(d) illustrates the design of the piston locater of resonator #2.

FIGS. 9(e) and 9(f) illustrate the design of the sapphire locatingdevice of resonator #2.

FIG. 10(a) is a cross-sectional view of a modified design of resonator#1.

FIG. 10(b) is an explode cross-sectional view of a sapphire rod holdingdevice used in modified resonator #1.

FIG. 10(c) is a top view of modified resonator #1.

FIG. 11 is a graph of the surface resistance of two phases of(T1Pb)SrCaCuO and Cu vs. temperature as determined in Example 1.

FIG. 12 is a graph of the surface resistance of YBaCuO vs. maximummagnetic field at various temperatures as determined in Example 2.

SUMMARY OF THE INVENTION

The present invention comprises a dielectric resonator apparatus whichoperates in TE0iN mode wherein i and N are integers greater than orequal to 1, having a dielectric element of sapphire positioned betweenand in contact with two discrete films of at least one superconductingmaterial, all encased in an outer enclosure having means for magneticdipole coupling, wherein the improvement comprises:

(a) means for positioning the sapphire dielectric relative to the outerenclosure;

(b) means for positioning relative to the outer enclosure, loading andunloading superconducting films;

(c) means for holding the sapphire dielectric and superconducting filmsin place during use;

(d) means for suppressing modes other than TE0iN; and

(e) means for adjustment of the magnetic dipole coupling.

The resonator comprises a sapphire and two substrates bearing a coatingof high temperature superconducting (HTS) material, hereinafter referredto as films or HTS films. The films are positioned relative to thesapphire to enable the coating to contact said sapphire. The structureof HTS film-sapphire-HTS film in the resonator can be modified, so themicrowaves will either be exposed to both the high temperaturesuperconducting material and the outer enclosure material, or it willonly be exposed to the high temperature superconducting material at theend planes.

The present invention also comprises a dielectric resonator apparatushaving a dielectric element of sapphire positioned between and incontact with two discrete films of at least one superconducting materialand a means for magnetic dipole coupling, wherein the improvementcomprises a means for moving the resonator to test the superconductingfilms at multiple discrete areas.

The advantages of the present invention are achieved by devices toobtain exact locations of resonator components, to retain the locationsof resonator components during the usage of the resonator in anelectrical circuit, to position, to load and to unload thesuperconducting films, to suppress undesirable modes, to adjust themagnetic dipole coupling, and to couple to an electrical circuit. Thesedevices permit very accurate and repeatable measurements. They alsoallow for easy assembly, and consequently, reduce the assembly time.

DETAILED DESCRIPTION OF THE INVENTION

Throughout the following description, similar reference numerals referto similar elements in all figures of the drawings.

The present invention comprises three variations of a sapphire resonatorfor measuring high temperature superconducting thin film parameters. Thefirst resonator design is shown in FIGS. 1(a) and 1(b) and canaccommodate two different drawer designs as shown in FIGS. 2(a) and2(f). The second resonator design is shown in FIGS. 7(a) and 7(b). Thethird design is shown in FIG. 10(a).

The first resonator is shown in FIGS. 1(a), 1(b) and 1(d). As shown inFIG. 1(d) a sapphire dielectric, conveniently a rod (5) is sandwichedbetween two films (17). FIG. 1(c) shows the film as a substrate (10)which is coated on one or both sides with superconducting materials(15). Substrate (10) must be a single crystal that has a lattice matchedto super-conducting material. As shown in FIG. 1(a) the resonator body(25) (outer enclosure) sits on top of the bottom plate (20) containingtapped holes (90) as shown in FIG. 1(b), for assembly screws. FIG. 1(b)shows the bottom film (17) is positioned on a drawer (40) which can bewithdrawn out of body (25) for film loading and unloading. In operation,FIG. 1(a) show that drawer (40) containing clearance holes (80) isattached to body (25) by two screws (75). As shown in FIG. 1(b) thesapphire rod (5) is centered in the resonator by a locating device 52.Piston (35) sits on top of sapphire rod (5) and holds the top film (17).A piston locater (52), located relative to the body (25) by pins (70),is used to guide the piston (35) to its exact location relative to thebody (25).

A holding device (50), described later, together with the top plate (30)will exert a pressure on the film-sapphire-film structure, and hold thestructure in place during the characterization process. Two connectorassemblies, FIGS. 1(b) and 1(e), each consist of a connector (95), atransition (60), a cable (55) in the shape of a loop at the end (notshown), one insulator (65) and a flange (68). The connector assembliescouple the resonator to an external electrical circuit. FIG. 1(b) showsshims or spacers (32) are used to adjust the coupling of eitherconnector. The shim (32) is inserted between the resonator body (25) andthe flange (68) on the connector to adjust the penetration of cable (55)into the resonator interior. All components of the resonator .are fixedin place by screws (87) shown in FIG. 1(b) inserted into tapped holes(85) shown in FIG. 1(a).

The sapphire rod used in this embodiment had a dielectric constant ε_(r)=9.2, and a loss factor (tan δ) of from 10⁻⁶ to 10⁻⁹ at cryogenictemperatures. Its C-axis must also be perpendicular to its end surfaceswithin ±1°.

Depending upon the method of locating the sapphire rod, there are twodifferent drawer designs. FIGS. 2(a) and 2(c) show the design of drawer#1. It can be made of any non-ferrous metal which has appropriateelectrical conductivity and thermal conductivity. The material used inthis embodiment is oxygen free copper. This drawer as shown in FIG. 2(a)and in cross-section in FIG. 2(c) consists of a first recess (100) tohold a film, a second recess (105) surrounding said first recess to holda microwave frequency absorber, two clearance holes (80) for two dowelpins (115) and as shown in FIG. 2(b) and two holes (102). Holes (102)are for screws (75) shown in FIG. 1(a) to attach the drawer to the bodyand are in an edge of the drawer perpendicular to the recesses. Thedepth of recess (100) is such that the surface of the film is slightlyabove the surface of the drawer, preferably 0.025 mm. The bottom plate,the top of the recess, and the bottom of the drawer must be parallel toeach other to within 0.07 mm, preferably 0.025 mm. The above surfacesmust also be perpendicular to axis "A" as shown in FIG. 2(c) within ±1°.The drawer #1 has chamfers (135) as shown in FIGS. 2(a) to ease itsinsertion into the body of the resonator.

The sapphire locater #1 to be used with drawer #1 is shown in FIG. 2(d).This locater is made of two non-metallic sheets (120) each with amaximum dielectric constant of about 4, and a low dielectric loss suchas Kapton®, or any polymer film. Its thickness should be chosen tocompromise for the mechanical strength and the dielectric loss. Thematerial used for this embodiment is a polyimide Kapton 300HN® film,0.076 mm thick. Sheet (120) has a clearance hole (130). Hole (130)permits positioning the sapphire dielectric relative to the outerenclosure by means of a dowel pin. Sheet (120) has an arc or notch (125)with a radius that matches the radius of the sapphire rod. Thus theshape and size of the notch are suitable to fit against the contour ofthe sapphire dielectric. The width of the sheet (120) is made to fit theslot in the body of the resonator which will be described later. Thelocater has chamfers (135) at the corners of the edge containing thenotch to ease its insertion into the body of the resonator. FIG. 2 (e)shows arc (125) in enlarged detail.

FIGS. 2(f) and 2(h) show the design of drawer #2. It is made of anon-ferrous metal which has appropriate electrical conductivity and agood thermal conductivity. The material used in this embodiment isoxygen free copper. Similar to drawer #1, it consists as shown in FIG.2(f) of a first recess (100) to hold a film, a second recess (105) tohold a microwave frequency absorber, two pins (110), two chamfers (135),and as shown in FIG. 2(g), two holes (102). The holes (102) are forscrews (not shown) to attach the drawer #2 to the resonator body (25) asshown in FIG. 1(a). The bottom, the top of the recess, and the bottom ofthe drawer must be parallel to each other within 0.076 mm, preferably0.025 mm. The above recesses are perpendicular to axis "B" as shown inFIG. 2(h).

The sapphire locater #2 to be used with drawer #2 is shown in FIG. 2(i). This locater is made of a rectangular non metallic sheet (145) witha maximum dielectric constant of about 4, and a low dielectric loss suchas Kapton®, or any polymer film. Its thickness is chosen to compromisefor the mechanical strength and the dielectric loss. The material usedfor this embodiment is a polyimide Kapton 300HN® film, 0.076 mm thick.Sheet (145) has an opening or hole (140) having a size and a shapethrough which a sapphire rod dielectric will pass with minimalclearance. Sheet (145) width is made to fit the slot in the body of theresonator which will be described later. The locater has chamfers (135)at two adjacent corners to ease its insertion into the body of theresonator.

FIG. 3(a) shows the design of the body of the resonator. At the centerof the body is a hole (165). Its diameter is determined such that,including all tolerances built into all components, only film is visiblewhen viewed from the top. No film edges are visible when viewed from thetop. Its diameter is also calculated proportionally to the sapphire roddiameter to maximize performance. The ratio between the diameter of hole(165) and the diameter of the sapphire rod must be greater than 2.0. Theratio used in this embodiment is 2.23. The surface created by hole (165)must be polished to reduce the energy loss caused by microwaveabsorption. Four tapped holes (85) are for assembly screws (87) aspreviously shown in FIG. 1(b). On top of hole (165) is a recess (155) asshown in FIG. 3(b). The size of this recess is larger than the size ofthe piston to create an even rectangular gap for helium gas to reach thecenter of the resonator. On the top surface, there are two dowel pins(70). In the front of the body of the resonator are two tapped holes(160). These are for screws (75) as shown in FIG. 1(a). There is a slot(150) at the bottom. For drawer #1, this slot extends through the entirelength of of the resonator. For drawer #2, this slot is only machinedpartially through. The width of this slot is precisely made to matchthat of the drawer to ensure the film and the sapphire are exactlylocated. FIG. 3(c) is a bottom view of the body of the resonator. Asshown in FIG. 3(d), hole (170) and tapped holes (175) are used to fastenthe connectors onto the body of the resonator.

The top plate (30) as shown in FIG. 1(b) is depicted in detail in FIGS.3(e) and 3(f). A slot (74) depth is calculated to ensure propercompression of the holding device. The top plate has two locating holes(79), four countersink holes (72), and a through hole (77) for heliumgas to enter the resonator. Holes (79) are for dowel pins (70) as shownin FIG. 3(b). Holes (72) line up with tapped holes (85) of FIG. 3(a).

The bottom plate as shown in FIG. 1(b) is depicted in detail in FIGS.3(g) and 3(h). It has four tapped holes (84) that are used to mount theresonator onto a supporting device (now shown) which is inserted into adewar (not shown). Four countersink holes (89) are used to bolt it tothe body of the resonator. There are two slots (82) for mounting severalheaters (not shown) to control and stabilize the resonator temperature.

The cable size to be used in this resonator is one of the factors thataffects the sensitivity and the accuracy of the measurements. The cableis depicted as element (55) in the connector assembly shown in FIG. 1(e). The performance of the resonator improves when small cable is used,because less case mode is developed. The cable used in this embodimenthas an 0.94 mm outer diameter. To use this cable with a commercialconnector such as Omni Spectra Flange Mount Cable Jack #1006-7985,having an inner diameter of 2.2 mm and an outer diameter of 3.07 mm,available from Omni Spectra, 740 4th Avenue, Waltham, Mass. 02254, atransition as shown in FIGS. 3(i) and 3(j) and an insulator as shown inFIGS. 3(k) and 3(m) are employed to achieve mating. The transition shownin FIGS. 3 (i) and 3 (j) is an electrical conductor tube. It is depictedas element (60) in the connector assembly shown in FIG. 1(e). Theoutside diameter of the transition fits the inside diameter of thecommercial connector, (for this embodiment 2.2 mm). The inside diameterof the transition fits the outside diameter of the cable (0.94 mm inthis embodiment). The insulator shown in FIGS. 3(k) and 3(m) is adielectric ring. It is depicted as element (65) in the connectorassembly shown in FIG. 1 (e). Its outside diameter is exactly the sameas that of the transition. Its inside diameter is smaller than thediameter of the conductor tube of the transition element to avoid ashort circuit between the transition and the center conductor of theconnector. The transition is soldered to the connector. The materialsused for the transition and the insulator in this embodiment are oxygenfree copper and Teflon®, respectively. The penetration of the cables(55) into the resonator is adjusted by inserting/removing shims (32) atthe connector flanges as shown in FIG. 1(b).

FIG. 4(a) shows the design of the piston. It is made of a non-ferrousmetal having appropriate electrical conductivity and thermalconductivity. The material used in this embodiment is oxygen freecopper. This piston consists of a first recess (185) to hold asuperconducting film, a second recess (190) to hold a microwavefrequency absorber, a through hole (180) for an externally appliedvacuum to reach the superconducting film, and as shown in FIG. 4(b) acounterbore (195) for placement of a holding device. FIG. 4(c) providesenlarged detail of the first recess (185). The depth of recess (185) issuch that the surface of the film is slightly above the surface of thedrawer. The bottom plate, the top of the recess, and the bottom of thepiston must be parallel to each other to within 0.076 mm, preferably0.025. The above surfaces must also be perpendicular to axis "C" asshown in FIG. 4(d) within ±1°. Elements (185), (190) and (180) are asdefined above for FIG. 4(a). The piston fits into the resonator outerenclosure with sufficient clearance to permit the passage of a coolinggas from an external source.

FIG. 4 (e) shows the design of the piston locater. This locator isdepicted as (52) in FIG. 1(b). It has two holes (200) that are used withdowel pins to position the piston locater relative to the body of theresonator. The size of slot (205) is large enough, so the piston candrop through with minimal clearance. The clearance between the pistonand the piston locater can be in the range from 0.025 mm to 0.15 mm. Inthis embodiment, all the dimensions are controlled to create at least0.025 mm clearance between them. The piston is detailed in FIG. 4(a) andis positioned as (35) in FIG. 1(b).

The performance of the resonator greatly depends on the placement of thesapphire rod and the films. There are several locating devices for thesapphire rod. Sapphire locaters #1 and #2 are shown in FIG. 2(d) andFIG. 2 (i). A film is either positioned in the resonator by pin-and-holefashion, or by confining it into place by any of the known prior artmethods such as by use of pins, recess, walls, and the like. The filmhas a hole at the exact location where the sapphire rod has to beplaced. The film can be designed to remain in or to be removed from theresonator before measurements are made. Sapphire locating devices #3 and#4, described hereinafter for resonator #2, can be employed in theresonator #1 if the body is modified in design to accommodate theselocating devices.

FIG. 5(a) shows a sapphire locating device #3. In this design, sapphirerod (5) is dropped roughly at the center of the resonator. There are aplurality of modified screws (210) with three non-metallic tips (215)which are glued, soldered, or fastened by any suitable means to thescrews. The non-metallic tips of the screws contact the sapphiredielectric. The modified screws turn, and move the rod to the center.The rod will be centered as the screws are stopped by the bottoms ofthreaded holes (220). Screws (210) will be removed or left in placeafter rod (5) is secured in place by a holding device. The non-metallicmaterial used for the tips (215) in this embodiment is sapphire.

Another sapphire locating device (#4) is shown in FIGS. 5(b), 5(c) and5(d). In this design, platform (223) has a perpendicular extensionhaving a recess (225). Sapphire rod (5) is fit into recess (225), and istemporarily held in place in the recess by an externally applied vacuumthrough hole (235). The sapphire locating device #4 is positioned on topof the body of the resonator by employing two dowel-pin-guiding-holes(230) as seen in FIG. 5(c). Once it is set, the vacuum is turned off torelease the sapphire rod (5), and the sapphire locating device #4 isremoved. The sapphire rod (5) is then secured by a holding device (notshown).

The sapphire locator devices #1 and #2 as shown in FIGS. 2(d) and 2 (i)may also be employed in resonator #2 if their shape and size is modifiedto fit the body of this resonator.

Several holding devices can be used to keep all the resonator componentsfrom moving during the operation. FIG. 6(a) shows a holding device whichis a system of helical compression springs (240) that are distributedevenly on top of the piston (35). These springs are attached to eitherpiston (35) or to the inside surface of top plate (30) by soldering orby other known methods. Once the top plate is screwed on to resonatorbody (25), the springs will be compressed to produce a desired holdingforce. The holding force can be adjusted by adding or removing springs,by selecting springs with higher tension rate, or by changing thedistance the springs are compressed.

Another holding device is shown in FIG. 6(b) comprising plate (245). Itis inserted between resonator top plate (30) and piston (35). Plate(245) material is selected such that it will compensate for all thedifferent thermal contractions of material involved in the resonator atcryogenic temperature, such as Teflon®. Thus, the final compressionforce on film (17) and rod (5) are very close to the force applied onthem before the resonator is cooled down.

The holding device used in the resonator of FIG. 1(b) is shown in FIG.6(c). A series of Belleville springs (50) (or disc springs) are stackedon top of piston (35). These springs compress the film-sapphire-filmstructure against the bottom plate when the resonator top plate (30) isscrewed on to body (25). The spring force is adjusted by adding orremoving springs, by selecting springs with higher tension rate, bystacking the springs in different configurations (parallel or seriesforms), or by changing the distance the springs are compressed. Thisholding device is compact because of the Belleville spring design. It isalso more stable than the holding device of FIG. 6(a) because the springforce is concentrated directly on top of the rod.

The microwave frequency absorber eliminates all modes but TE0iN (whereini and N are each integers greater than or equal to 1, preferably 1, 2,or 3, and most preferably i and N are both 1) modes because TE0iN modescarry only circular currents. The absorber can be any ferrite material.The absorber used in this embodiment is ECCOSORB® (available fromEmerson & Cuming Inc., Woburn, Mass. 01888). The absorber is located inrecess 105 as shown in FIG. 2(a).

Once the resonator body in FIG. 3(a), the bottom plate in FIG. 3(g) andthe two connectors of FIG. 1(e) are fastened together (as shown in FIGS.1(a) and 1(b)), there are several steps required to assemble theremainder of the resonator. If drawer #1 and sapphire locater #1 areused, the first step is to position the bottom film (17), and sapphirerod (5) (FIG. 1(d)) in the resonator. This step is carried out byplacing the film in the recess (100) of drawer #1 (FIG. 2(a)). Themicrowave frequency absorber is placed in recess (105) of drawer #1(FIG. 2(a)). Two sapphire locaters #1 (FIG. 2(d)) are then placedopposite to each other on drawer #1. At this time, drawer #1 is slid inand bolted on resonator body (25) using screws (75) depicted in FIG.1(a). Next, dowel pins (115) (FIG. 2(b)) are inserted through holes(130) (FIG. 2(d)) of sheet (120) and holes (80) (FIG. 2(a)) of drawer #1to locate exactly the circle formed by arcs (125) (FIG. 2(d)) relativeto resonator body (25) (FIG. 1(b)). The sapphire rod (5) is positionedinto the circle formed by arcs (125). The second step is to position thetop film (17) in the resonator. To accomplish this step (FIG. 1(b)), thepiston locater (52) is located on top of body (25), with the help ofdowel pins (70). The film (17) is then dropped into recess (185) of thepiston (FIG. 4(a)). After covering the hole (180) with a suction device,piston (FIG. 4(a)) is picked up, turned upside down and guided into theresonator at position (35) in FIG. 1(b)). Then, the suction device andthe piston locater are removed. The last step is to place a holdingdevice (FIGS. 6(a), 6(b) and 6(c)) in recess (195) of the piston (FIG.4(b)), put the top plate (30) (FIG. 1(b)) on top of resonator body (25),and tighten screws (87). When the resonator is ready for testing, dowelpins (115) and sapphire locaters #1are removed from body (25). Shims(32) will be added or removed to adjust the coupling strength of eitherconnector.

If drawer #2 and locater #2 are used, the assembly can be followed usingFIGS. 1(a), 1(b), 1(c), 1(d), 1(e), 2(c), 2(d) and 4(a). The first stepis to position the bottom plate (20) and the connectors (95) (FIG. 1(e))to the body (25). The second step is to position the bottom film (17),and sapphire rod (5) in the resonator. This step is carried out byplacing the film in the recess (100) of drawer #2 (FIG. 2 (f)). Sapphirelocater #2 (FIG. 2(i)) is then placed on drawer #2. Sapphire locater #2is confined by the walls of the resonator body (25) and pins (110). Thesapphire rod (5) is positioned into circle (140). The third step is toposition the top film (17) in the resonator. To accomplish this step,the piston locater (52) (FIG. 4 (e)) is set on top of body (25) (FIG.1(b)), with the help of dowel pins (70). The film (17) is then droppedinto recess (185) of piston (35), FIG. 4(a). After covering the hole(180) with a suction device, piston (35) is picked up, turned upsidedown and guided into the resonator. Finally, the suction device and thepiston locater are removed. The last step is to place a holding devicein recess (195) (FIG. 4(b)) of the piston, and as depicted in FIG. 1(b),put the top plate (30) on top of body (25), and tighten screws (87).Shims (32) will be added or removed to adjust the strength of thecoupling.

This invention further comprises a sapphire resonator of a seconddesign, denoted sapphire resonator #2. The design of this resonator isshown in FIGS. 7(a) through 7(e). FIG. 7(d) shows a sapphire dielectric(5), conveniently a rod, is sandwiched between two films (17). FIG. 7(e)shows the film (17) as a substrate (10) which is coated on one or bothsides with superconducting materials (15). Substrate (10) must be asingle crystal that has a lattice matched to the superconductingmaterial. As shown in FIGS. 7(a) and 7(b) the resonator body outerenclosure (295) sits on top of the bottom plate (305). The sapphire rod(5) is centered in the resonator by a locating device (260) to bedescribed later. As seen in FIG. 7(b) piston (290) sits on top ofsapphire rod (5) and holds the top film (17). A piston locator (285), islocated relative to the body by pins (250). A holding device (50),described later, together with the top plate (280) will exert a pressureon the film-sapphire-film structure and hold the structure in placeduring use, i.e., the film characterization process. Two connectorassemblies, as shown in FIGS. 7(b) and 7(c), each consisting of aconnector (95), a transition (60), one cable (55), and one insulator(65), will couple the resonator to an electrical circuit. As shown inFIG. 7(b) shims (292) are used to adjust the strength of the coupling ofeither connector. All components of the resonator are fixed in place byscrews (265).

FIG. 8(a) shows the design of body of resonator #2 depicted as (295) inFIGS. 7(a) and 7(b). At the center is a hole (320). Its diameter iscalculated proportionally to the sapphire dielectric diameter,conveniently a rod, to maximize the performance. The ratio between thehole (320) diameter and the diameter of the sapphire rod must be greaterthan 2.0. The ratio used in this embodiment is 3.3. The surface createdby hole (320) is polished to reduce the energy loss caused by microwaveabsorption. There are two dowel pins (250) for guiding the top plate andthe piston locater. Three tapped holes (310) are used to attach the bodyto the bottom plate. There is an engraved line (325) to indicate theorientation of the bottom film. FIG. 8(b) shows a side view of the bodyof resonator #2. Three tapped holes (315) are for entry of the sapphirelocater devices. FIG. 8(c) shows a different side view of the body ofresonator #2. Tapped holes (330) and hole (335) are used to attachconnectors to the body.

FIGS. 8(d) and 8(e) show the design of the top plate of resonator #2depicted as (280) in FIG. 7(b). It has a counterbored hole (340) toclear the piston diameter, two clearance holes (345), and threecountersink holes (342). Holes (345) are for pins (250) in FIGS. 7(a)and 7(b). Holes (342) are for screws (265) shown in FIG. 7(a).

FIGS. 8(f) and 8(g) show the design of the bottom plate depicted as(305) in FIG. 7(b). Similar to resonator #1, bottom plate (305) has fourtapped holes (350) as shown in FIG. 8(f) to mount the resonator onto asupporting device (not shown), which is inserted into a dewar flask (notshown). There are three countersink holes (380) as shown in FIG. 8(f).Holes (380) are for screws (265) as shown in FIG. 7(b). As shown in FIG.8(g) the bottom plate also has one recess (355) to locate asuperconducting film (not shown), and another recess (370) for microwaveabsorber (not shown). The depth of recess (355) is such that the surfaceof the film is slightly above surface "A" as shown in FIG. 8(g),preferably 0.025 mm. The bottom of the recess, surface "B" as shown inFIG. 8(g), and surface "A" must be parallel to each other within 0.076mm, preferably 0.025 mm. The above surfaces must also be perpendicularto axis "D" as shown in FIG. 8(g) within ±1°. To center the film in theresonator, diameter (360) is made to slip fit to the body of theresonator. Diameter (365) is smaller than diameter (360) to create acircular gap; so microwaves can reach the microwave absorber painted inthe recess (370). This gap could range from 0.254 mm-1.016 mm (total),and preferably is 0.508 mm. Two slots (375) are used to mount severalheaters to control and stabilize the resonator temperature.

FIGS. 9(a), 9(b) and 9(c) show the design of the piston. The piston isdepicted as (290) in FIG. 7(b). The piston has a recess (390) for asuperconducting film (not shown), a recess (395) for microwave absorber(not shown), a through hole (400) for vacuum to reach the film, and anengraved line (405) to indicate the orientation of the film. The depthof recess (390) is such that the surface of the film is slightly abovethe surface of the piston recess (390), preferably 0.025 mm. Diameter(385) is made smaller than the hole of the body of the resonator (shownas (320) in FIG. 8(a)) to create an even circular gap for cooling gassuch as helium to reach the center of the resonator. A counterbore (402)is used for placement of a holding device (not shown). The bottom of therecess (390), the top of the recess (390), and the bottom surface (398)of the piston must be parallel to each other within 0.076 mm, preferably0.025 mm. The above surfaces must also be perpendicular to axis "E" ofthe piston, shown in FIG. 9(b) within ±1°.

FIG. 9(d) shows the design of the piston locater. This is depicted as(285) in FIG. 7(b). It is a plate of either plastic or metal which has ahole (415) through which the piston will pass with minimal clearance toguide the piston, and two clearance holes (410). Holes (410) are forpins (250) shown in FIG. 7(b).

FIGS. 9(f) and 9(e) show the design of a sapphire rod locating devicewhich is a modified shoulder screw (210) with a sapphire rod (215) atthe end. The rod is inserted into the screw and held in place byadhesive or by other known method. This locating device is depicted as(260) in FIG. 7(a).

Any of the holding devices shown in FIGS. 6(a), 6(b), or 6(c) can beemployed with resonator #2. Resonator #2 uses identical transition shownin FIGS. 3(i) and 3(j) and insulator shown in FIGS. 3(k) and 3(m) as inresonator #1. The penetration of the cables into the resonator isadjusted by adding or removing the shims (292) as shown in FIG. 7(b).

There are two major differences between resonator #1 and resonator #2.Unlike resonator #1, which does not allow the microwaves to be exposedto the material of the drawer, a superconducting film of resonator #2 issmaller than surface "A", FIG. 8(g); so microwaves will be exposed toboth the superconducting material as well as the platform material. Theother difference is the height of the sapphire rod whereas the height is2.5 mm in resonator #1, it is 2 mm in resonator #2.

The assembly of resonator #2 can be followed using FIGS. 7(a), 7(b),7(d), 8 (f), 8(g), 9(a), 9(b) and 9(c). The first step is to fasten thebottom plate (305) and the connectors (95) to the body (295) (FIGS. 7(b)and 7(c)). The second step is to place film (17) in the recess (355) ofthe bottom plate FIG. 8 (f). Next, screw subassemblies (260) (FIG. 7(a))are partially turned into the body (295). At this time, the sapphire rod(5) is placed roughly at the center of body (295). Then, the screwsubassemblies are turned until they are stopped. During that time, thesapphire rod is moved to the exact center of the body. The next step isto position the top film (17) in the resonator. To accomplish this step,first, the piston locater (285) is set on top of body (295), with thehelp of dowel pins (250). The superconducting film is then dropped inrecess (390) of the piston, FIG. 9(a). After covering the hole (400)(FIG. 9(c)) with a suction device, the piston is picked up, turnedupside down and guided into the resonator. Then, the suction device andthe piston locater are removed. The last step is to place a holdingdevice, FIG. 6(c), in recess (402) (FIG. 9(c)) of the piston, put thetop plate (280) (FIG. 7(b)) on top of the body, and tighten screws(265). When the resonator is ready for testing, the screw subassembliesare taken out. Shims (292) will be added or removed to adjust thestrength of the coupling of either connector.

The designs of resonator #1 and #2 can be modified to test any filmsizes, provided related dimensions such as film recesses, overallresonator dimensions, and sapphire rod dimensions are changed toaccommodate for the new films and to assure the performance of theresonator is not affected.

The present invention further comprises a third variation of a sapphireresonator which is a modification of resonator #1. The apparatus ismodified to determine the local surface resistance and the uniformity ofa superconducting thin film on a large substrate as shown in FIGS.10(a), 10(b), and 10(c). A large high temperature superconducting thinfilm can be tested at various discrete areas. This design has twosubassemblies. The first assembly, shown in FIG. 10(a), has acylindrical plate (460) that holds a wafer with superconducting film(455) on its underside. A vacuum hole (490) keeps film (455) fromfalling during the operation. Plate (460) is rotated around its center Zaxis manually or mechanically, or by any other rotating device. Thesecond subassembly is called a testing head. This subassembly includesthe rest of components of the resonator as shown in FIG. 10(a). Asapphire dielectric (5), conveniently a rod, and a small hightemperature superconducting film (17) are sandwiched between plate (460)and a piston (475). Body (445) is used to hold cables (450). Body (445),piston (475), and bottom plate (440) are held and guided by shoulderscrews (480). Holding device (435), shown in FIGS. 6(a), 6(b) and 6(c),is used to keep all components together during operation. Rod (430) iswelded, or screwed, or fastened by any equivalent method, to bottomplate (440) of the resonator. The other end of rod (430) is connected toan actuator (420). The actuator can be any device that reciprocates rod(430) up and down in the Z direction perpendicular to plate (460), andmoves the testing head in the X direction as depicted in FIG. 10(c).FIG. 10(b) is an enlarged view that shows a nonmetallic sheet (485) usedto center and to keep sapphire rod (5) from moving during operation.This sheet must have a low dielectric constant, and a low dielectricloss such as Kapton® or any polymer film. The testing head is connectedto another actuator. The actuator can be an air cylinder, a lineractuator, or any device that moves the testing head in the X direction.

The design of piston (475) is identical to the piston describedpreviously in FIG. 4(a) except for additional tapped holes for shoulderscrews (480). The design of bottom plate (440) is identical to the platedescribed previously in FIG. 3(g) except for additional holes forshoulder screws (480). Sheet (485) is identical to sapphire locator #2(FIG. 2 (i)) except its size is made to fit the recess of body (445).

In operation, as shown in FIGS. 10(a), 10(b) and 10(c), the testing headis moved to a desired location on the film (455). This is accomplishedby the combination of the movement of the testing head in the Xdirection and the rotation of the plate (460) around its Z axis. Then,the actuator (420) moves the testing head in the +Z direction,compressing a holding device (435), previously described. At this time,film (17), sapphire rod (5), and film (455) forms a resonator similar toresonators #1 and #2. Film parameters at this location are recorded.

To test another location on film (455), actuator (420) lowers thetesting head to keep the sapphire rod from touching film (455). Thetesting head and plate (460) are again moved to position the testinghead immediately below the desired testing location. The next stepsdescribed in the previous paragraph are then repeated.

The invented apparatus are used for measuring the parameters such assurface resistance, R_(s), critical current density, J_(c), and criticalrf magnetic field, H_(c), for high temperature superconductors (HTS).

For a given TE₀₁₁ mode HTS-sapphire-HTS resonator, the surfaceresistance, R_(s), is inversely proportional to the unloaded Q-value,Q₀, of the resonator:

    R.sub.s =G/Q.sub.0                                         (1)

Here G is the geometry factor, which can be either calculated by usingthe formulas given in Shen et al., 1992IEEE-MTT-S InternationalMicrowave Symposium Digest, pp. 193-196 or calibrated by a conductorsuch as copper with known R_(s). Therefore, the measurement of R_(s) issimply to measure the Q₀ of the resonator, which is known in the art.

In a TE₀₁₁ mode resonator, the maximum rf magnetic field, H_(max), andthe corresponding maximum current density, J_(max), in the HTS films arerelated to the R_(s) and the dissipated power, P₀, in the resonator asfollows: ##EQU1## Here J₁ (x) is the 1st order Bessel function of the1st kind; J₁,max =0.582 is the maximum value of the J₁ (x); ε_(r) =9.32and tanδ are the relative dielectric constant and the loss tangent ofsapphire, respectively; R is the ratio of the electrical energy storedinside to that stored outside of the sapphire rod; a and L are theradius and the length of the sapphire rod, respectively; ζ₁ is thetransverse wave number in the sapphire rod; λ is the wave length; λ_(d)is the penetration depth ρ is the radial dimension variable and dρ isthe differentiation of ρ. For a given resonator, all these quantitiesare constants. Therefore, the H_(max) and J_(max) can be determined bymeasuring the R_(s) and P₀ (P₀ can be measured by using a power meter).The resonators of the present invention provide measurements of improvedaccuracy and are reliably reproducible. For example, resonator #1 withdrawer design #2 provided reproducibility at a level of 2%. Theresonators of the present invention are easy to assemble while retainingsuperior sensitivity. Thus the apparatus are useful as a quality controltool for monitoring superconducting thin film manufacturing processes.

The critical magnetic field, H_(c), can be determined by the H_(max)value at which the R_(s) value exceeds certain selected criteria. TheJ_(c) is simply equal to H_(c) /λ_(d).

EXAMPLE 1

The surface resistance, R_(s), of the superconductor T1PbSrCaCuO (1212)and (1223) phases was measured using resonator #1 with drawer #2assembled as previously described. The data measurements were performedin a liquid helium storage dewar with a glass epoxy insert. Theresonator assembly was mounted on a copper plate at the end of astainless steel probe. The assembly was then evacuated and lowered intothe insert. Temperature was controlled with a pair of 100 W heaters. AHP-8510 vector network analyzer with 1 Hz frequency resolution was usedfor the measurements. A HP-8449A preamplifier and a Hughes 8030H02F TWTpower amplifier with output power up to 30 W were inserted at the inputof the resonator for the high powdered measurements. The R_(s) wasmeasured at 27 GHz. The resulting data are shown in FIG. 11 whichdepicts how the surface resistance of the superconductor T1PbSrCaCuO(1212) and (1223) phases, as well as a conducting Cu reference, varywith temperature. It is plotted at 10 GHz, according to f² low as afunction of temperature. The copper (Cu) data are for reference.

EXAMPLE 2

The surface resistance, R_(s), of the superconductor YBa₂ CuO_(7-a)wherein a is less than 1 was measured using resonator #1 with drawer #2assembled as previously described using the procedure of Example 1except that the R_(s) was measured at 5.55 GHz. The resulting data forsurface resistance at various temperatures are shown in FIG. 12 graphedas a function of maximum rf magnetic field. The critical magnetic fieldH_(c) is shown as the maximum magnetic field value at which the R_(s)value exceeds certain selected criteria.

What is claimed is:
 1. A dielectric resonator apparatus, operating inTE_(0iN) mode, where i and N are integers greater than or equal to 1,having a dielectric element of sapphire positioned between and incontact with two discrete films of at least one superconductingmaterial, all encased in an outer enclosure having a means for magneticdipole coupling connected to an electrical circuit, said means formagnetic dipole coupling held in place by and extending through saidouter enclosure and said means for magnetic dipole couplingcharacterized by a magnetic dipole coupling strength relative to saidelectrical circuit, wherein the improvement comprises:(a) means forpositioning the sapphire dielectric relative to the outer enclosure andbetween the two discrete films; (b) means for positioning the twodiscrete films relative to the outer enclosure and relative to thesapphire dielectric; (c) means for holding the sapphire dielectric andthe two discrete films in contact during operation; (d) means forsuppressing a mode other than TE₀₁₁, said means for suppressing locatedinside the outer enclosure and outside a cavity, said cavity beingdefined as the space existing between the two discrete films and betweenthe sapphire dielectric and the outer enclosure; and (e) movable meansfor adjustment of the strength of the magnetic dipole coupling to anelectrical circuit, located between the two discrete films and insidesaid cavity, but without contacting the two discrete films and thesapphire dielectric.
 2. The resonator of claim 1 wherein the means forpositioning the sapphire dielectric comprises two non-metallic sheetseach having (1) a notch centered on one edge, said notch having a sizeand a shape suitable to fit against the sapphire dielectric, (2) a holeto permit positioning the sapphire dielectric relative to the outerenclosure with a dowel pin, and (3) chamfers at corners of the edgecontaining the notch.
 3. The resonator of claim 1 wherein the means forpositioning the sapphire dielectric comprises a rectangular non-metallicsheet having (1) an opening of a size and a shape through which thesapphire dielectric will pass with minimal clearance and (2) chamfers attwo adjacent corners.
 4. The resonator of claim 2 or 3 wherein the sheetcomprises a polymeric film having a dielectric constant with a maximumvalue of
 4. 5. The resonator of claim 2 or 3 wherein the sheet iscomprised of polyimide Kapton 300HN.
 6. The resonator of claim 1 whereinthe means for positioning the sapphire dielectric comprises a pluralityof screws inserted through openings in the outer enclosure, said screwshaving non-metallic tips, said tips contacting the sapphire dielectric.7. The resonator of claim 6 wherein the non-metallic tips comprisesapphire tips.
 8. The resonator of claim 1 wherein the means forpositioning the sapphire dielectric comprises (1) a device comprising aplatform to which is attached perpendicular to the platform an extensionhaving a recess into which the dielectric fits, (2) a hole runningthrough said extension and platform for application of an externalvacuum to hold the dielectric in place in the recess, and (3) at leasttwo additional holes and dowel pins for positioning the device relativeto the outer enclosure, each of said additional holes used to guide saiddowel pins.
 9. The resonator of claim 1 wherein the means forpositioning the superconducting film comprises a drawer of non-ferrousmetal having (1) a first recess for holding the superconducting film,(2) a second recess for holding a microwave frequency absorbersurrounding said first recess; (3) at least two holes for dowel pins ina surface of the drawer having said recesses; and (4) at least two holesfor screws in a surface of the drawer perpendicular to said recesses.10. The resonator of claim 1 wherein the means for positioning thesuperconducting film comprises (1) a piston having(a) a first recess tohold the superconducting film, (b) a second recess to hold a microwavefrequency absorber,(c) a hole running through said piston forapplication of an external vacuum to hold the said film in place, and(d)a counterbore, and (2) a piston locator in the form of a flat sheetcontaining an opening through which the piston will pass with minimalclearance and at least two holes, one on each side of said opening, anddowel pins used in said holes for positioning said piston locatorrelative to the outer enclosure.
 11. The resonator of claim 10 whereinthe piston has a diameter such that the piston will fit into the outerenclosure with sufficient clearance to permit passage of a cooling gasfrom an external source.
 12. The resonator of claim 1 wherein the meansfor holding the sapphire dielectric and superconducting film in placeduring operation comprises at least one helical compression springsecured to a top plate of the outer enclosure, said spring beingcompressed when the top plate is in place.
 13. The resonator of claim 1wherein the means for holding the sapphire dielectric andsuperconducting film in place during operation comprises at least onehelical compression spring secured to a piston holding thesuperconducting film, said spring being compressed when the top plate isin place.
 14. The resonator of claim 1 wherein the means for holding thesapphire dielectric and superconducting film in place during operationcomprises a plate of material which compensates for thermal contractionsof the resonator at cryogenic temperature.
 15. The resonator of claim 14wherein the plate is comprised of Teflon®.
 16. The resonator of claim 1wherein the means for holding the sapphire dielectric andsuperconducting film in place during operation comprises a plurality ofdisc springs stacked between the means for holding the superconductingfilm and an inside surface of a top plate of the outer enclosure. 17.The resonator of claim 1 wherein the means for suppressing undesirablemodes comprises a microwave absorber contained in a recess in the meansfor positioning the superconducting film which surrounds thesuperconducting film.
 18. The resonator of claim 1 wherein the means foradjusting the strength of the magnetic dipole coupling comprisesadjusting penetration into the resonator enclosure of a cable extendingfrom at least one connector by inserting or removing shims between theresonator body and a flange on said connector employed for the magneticdipole coupling.
 19. The resonator of claim 1 further comprising saidmeans for connecting the resonator to an electrical circuit which is aconnector having (1) a transition element comprising an electricalconductor tube having an outer diameter which is inserted into acommercial connector and an inner diameter into which is inserted acable having an outside diameter of a maximum of 0.94 mm, said cablecomprising said means for magnetic dipole coupling, and (2) an insulatorcomprising a dielectric ring having an outer diameter equal to the outerdiameter of the transition element and an inner diameter less than thediameter of the conductor tube of the transition element.
 20. Theresonator of claim 19 wherein the transition element is comprised ofoxygen free copper and the insulator is comprised of Teflon®.